1. Field of the Invention
This invention relates to a multiple speed load hoisting system. More specifically, it relates to such a system with fluid pressure for hoisting a load, a manual control for shifting from a low lifting speed to a high lifting speed, and a safety device, that is responsive to excess fluid pressure at a high lifting speed, for automatically cancelling the influence of the manual control and allowing reversion to a low lifting speed.
2. Description of the Prior Art
Fluid cylinders and fluid motors have been used as components for hoisting loads at multiple speeds. Such components usually have two different effective areas that can be utilized for hoisting. A maximum effective area is provided for hoisting heavy loads at low speed, and a smaller effective area is provided for hoisting lighter loads at higher speed. A manual control enables an operator to shift from low speed to high speed, but a problem results when the load being hoisted is greater than the maximum design load for high speed operation. Fluid pressure within the components can be manified to the point of component failure by such shifting of the manual control.
Some fluid circuits for operating multiple speed hoisting components, such as fluid cylinders and fluid motors, have a pressure relief valve, that is located between the component and a holding valve, to prevent the component pressure from climbing too high. While this relief valve protects the components from high pressures, it allows fluid to escape, from the relief valve back to a sump tank, and thus, lowers the load being hoisted.